1. Field of the Invention
The present invention relates to an optical amplifier apparatus using an optical amplifying fiber.
There is known an optical amplifier apparatus including an optical amplifying fiber which directly amplifies optical signals. This optical amplifier apparatus is provided with a laser for producing pumping light. If the laser deteriorates, the input signal light cannot be amplified to the predetermined level. Therefore, the construction is employed where a stand-by laser is provided and that stand-by laser is switched to when the working laser deteriorates.
2. Description of the Related Art
An optical amplifying fiber generally is comprised of SiO.sub.2 -GeO.sub.2 as a host glass to which is doped erbium (Er) and has an amplifiable wavelength band of 1.55 .mu.m. Therefore, since this matches with the minimum loss wavelength band of the quartz optical fiber used for usual optical signal transmission, this is applied to the optical amplifier apparatus in an optical signal transmission system. The wavelength of the pumping light may be made 0.51 .mu.m, 0.66 .mu.m, 0.81 .mu.m, 0.98.mu., and 1.48 .mu.m, so there is known a construction where a semiconductor laser of a 0.98 .mu.m or 1.48 .mu.m band is used as the pumping light source of the erbium-doped optical amplifying fiber. Further, there is known a construction wherein aluminum (Al) or phosphorus (P) doped AL.sub.2 O.sub.3 -GeO.sub.2 -SiO.sub.2 or Al.sub.2 O.sub.3 -P.sub.2 O.sub.5 -GeO.sub.2 -SiO.sub.2 is used as the host glass and erbium is doped. Note that there is also known a construction wherein neodium (Nd), praseodymium (Pr), and other rare earth elements are doped instead of erbium.
The conventional optical amplifier apparatus using this optical amplifying fiber, as explained in detail later with respect to FIG. 1, includes an optical amplifying fiber, a working laser, a stand-by laser, a signal light monitor circuit, a drive current monitor circuit, laser driver circuits, photocouplers, an input optical fiber, an output optical fiber, and a photodiode for monitoring the output signal light.
Here, the above-mentioned drive current monitor circuit transforms the drive current of the working laser given by the laser driver circuit into voltage through resistors etc. and monitors the result. When the working laser deteriorates, there is a tendency for an increase of the drive current, so if the drive current of the working laser exceeds a predetermined value, it is decided that the working laser has deteriorated. The laser driver circuit of the stand-by laser is then placed in the active state, and the operation of the laser driver circuit of the working laser is stopped, whereby the working and stand-by systems are switched.
Note that as a similar technique for judging the above deterioration, in the normal Optical signal stabilization structure where the backward output light of the semiconductor laser is monitored and the optical output of the semiconductor laser is stabilized based on the results, the semiconductor laser may be judged to have deteriorated when the drive current of the semiconductor laser exceeds a set value. For example, this is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-215133.
As mentioned above, in the prior art, a laser was judged as having deteriorated by the rise of the drive current. Therefore, there was the defect that when the drive current of the laser became zero or fell remarkably due to a fault or line disconnection of the laser driver circuit, the laser was judged normal. Further, when the input signal light level fell by a large amount, it was necessary to make the pumping light power larger, so there were cases where the drive current of the laser would exceed a predetermined value. In those cases, there was the defect of a mistaken judgement as to laser deterioration.